This invention relates to a power supply comprising:
an input for receiving an input voltage, and an output for connecting a load, a coil, PA1 switching means for establishing a conduction path between the input terminal and the coil during an on-time and for interrupting the conduction path between the input terminal and the coil during an off-time, PA1 means for sustaining a conduction path for the current through the coil upon interruption of the conduction path between the input terminal and the coil, PA1 variation means for varying the ratio between the on-time and the off-time of the switching means in response to a difference between an output voltage across the load and a first reference voltage. PA1 reducing the ratio if the output voltage is larger than the first reference voltage, PA1 increasing the ratio if the output voltage is smaller than a second reference voltage whose value is smaller than the first reference voltage and PA1 maintaining the ratio constant if the output voltage lies between the first reference voltage and the second reference voltage. PA1 increasing the ratio from a first value to a second value and increasing the first value by an amount smaller than the difference between the first and the second value, if the output voltage is smaller than the second reference voltage, PA1 changing the on-time from the second to the first value as soon as the output voltage exceeds the second reference voltage, and PA1 interrupting the conduction path between the input terminal and the coil if the output voltage is smaller than the second reference voltage for a second time interval. By increasing the ratio from a first to a second value it is possible to obtain a rapid response to a load increase, for example, by making the second value significantly larger than the first value. If the load is too high, which manifests itself in that the output voltage remains below the second reference voltage for a second time interval, the power supply will disable itself. Since the current can only increase to the maximum extent for a time which is limited to the second time interval an effective current limitation is achieved. As a result, no current sensing is necessary to limit the current through the coil. In the known power supply current sensing is effected by measuring the voltage drop across a resistor through which the current to be measured flows. This resistor dissipates power. Since this resistor can now be dispensed with the efficiency of the power supply increases in comparison with that of a power supply which uses current sensing. PA1 means for generating clock pulses, PA1 storage means for the storage of numbers, which numbers represent numbers of clock pulses, PA1 counting means for counting clock pulses and for supplying a stop signal when a given number of clock pulses is reached, PA1 voltage comparison means for supplying a comparison signal in response to the comparison of the output voltage with at least one of the first, second and third reference voltages, PA1 control means for in response to the stop signal,
Such a power supply is generally suitable for supplying an output voltage to a load, which voltage is maintained at a desired value.
The invention further relates to a transmitter comprising a power output stage having a power supply terminal coupled to a controllable power supply for controlling the power of said power output stage in response to a power control signal.
Such a power supply is known from Electronic Design, Vol. 40, No. 21, Oct. 15, 1992, p. 104, which describes a power supply which controls the output voltage in the continuous conduction mode by varying the ratio between the on-time and the off-time of the switching means. Continuous conduction mode means that the current through the coil does not become zero.
A drawback of such a power supply is that if the load varies hardly or not at all and, as a consequence, the output voltage also varies hardly or not the output voltage is yet maintained by controlling the ratio between the on-time and the off-time. This control consumes current in a situation in which no control is necessary, resulting in unnecessary dissipation of power in the power supply. As a result, the efficiency of the power supply is lower than is strictly necessary.